MOSFET and IGBT switching devices suitable for high power applications such as electric motor control generally control the flow of power from sources of relatively high electrical potential in the range of hundreds of volts. These switching devices require gate-to-source turn-on potentials in the range of about 10 volts. Standard high side switch controllers for these switching devices generate the required 10 volt gate-to-source potential with a bootstrap capacitor. A charging circuit charges the bootstrap capacitor through a load for the switching device through a diode. This charging circuit is suitable for applications in which the switching device uses pulse width modulated (PWM) switching of a load with a reasonable value of resistance. In such applications, the charging circuit recharges the bootstrap capacitor during the OFF state intervals of the PWM switching cycle.
If the load is not present or it has a very high value of resistance the charging circuit cannot properly keep the bootstrap capacitor charged. If the switching device is switched on statically (without PWM), there is no OFF state interval in which to charge the bootstrap capacitor. In such instances, the bootstrap capacitor discharges and the switching device remains off.